The present invention relates to a radio digital signal transmission system and, more particularly, to a fault detection signal transmission system in which a transmit terminal sends to a receive terminal a fault detection signal which is representative of a fault in an input stage circuit of the transmit terminal.
A conventional fault detection signal transmission system of this kind comprises a transmit terminal and a receive terminal. In a digital signal processing circuit of the transmit terminal, a signal to be transmitted is digitally processed and then multiplexed with a parity check signal and other various kinds of signals including a fault detection signal. The fault detection signal is associated with an input stage circuit of the transmit terminal, i.e., a transmit interface circuit and any circuitry preceding the circuit as will be described hereinafter. The processed and multiplexed signal is modulated and frequency converted by a transmitter and sent o the receive terminal.
In the receive terminal, a received signal including the parity check signal and the fault detection signal is frequency converted and demodulated by a receiver, parity checked and demultiplexed by a digital signal processing circuit. Also, an alarm signal is provided based on the demultiplexed fault detection signal.
In such a transmission system, some time slots must be assigned to multiplex the fault detection signal as well as the parity check signal. This complicates the transmit terminal and lowers the transmission efficiency as will be more obvious hereinafter.